5.3 misfire on cylinder 8 on throttle lift - SOLVED

[zinc]

I'm chasing a weird misfire on my 2018 Tahoe with the L83/5.3 with 125k miles.

It started off with a failed coilpack that I was able to diagnose from a misfire code, swapping coil packs and checking misfire counts on an ODB reader. The failed coil pack was producing hundreds/thousands of misfires and very rough running, but after replacing the coil pack it was back running smooth again.

I did notice that there was still a much lower number of misfires on cylinder 8 still, probably in the region of 10-30 counts but it wasn't noticeable in the cabin and not enough to throw a code.

Since I was about to go on a 6k road trip I ended up replacing all 8 coil packs, replaced all spark plugs and wires as they were probably due anyway, and also went down a rabbit hole of checking fuel trims which I suspected leaky injectors and ended up replacing all 8 injectors and the fuel rails as they bent when I pulled them.

When I pulled the number 8 injector it was very black and looked like a bunch of carbon deposits on there compared to others so suggests this cylinder might have been running slightly rough for a while (see attached showing injectors 6 and 8).

Next noticed that cleaning the MAF seemed to improve the misfire counts and ended up replacing the MAF with a brand new AC Delco which seemed to fix the issue for a short while and thought we were good.

Sometime after that it slowly started to return, can't recall exactly but now after a 6k mile road trip, I have the following symptoms:

Reading misfire counts when running, I can see that on cold start there are a random misfires on cylinders 1-7 with counts between 1 and 4, but cylinder 8 is the worst and will see misfire counts up to 20ish.

Once running and warm, there are no misfires EXCEPT on cylinder 8, and this only occurs consistently when letting off the gas pedal or sometimes slowing down at light and the count is almost always 2, but sometimes 4 or 6. The misfire doesn't happen with the throttle open at all, only seems to occur when you snap the throttle closed after being open a decent amount e.g. on the highway when you lift.

Seemed like a vacuum leak to me, vacuum should be highest when the throttle is closed after being open. I smoke tested, replaced intake gaskets and throttle gaskets and no change.

Thought it might be a warped or cracked intake manifold, so replaced that with a new OEM part, torqued the intake bolts on a cold engine, first run showed absolutely ZERO misfires across the board and ran perfect. Thought that had fixed it for good but after that first heat cycle the misfires returned exactly as before and the bolts needed re-torquing again.

In total so far I have:
Replaced all coil packs
Replaced all spark plugs and wires
Replaced all injectors and fuel rails
Replaced intake gaskets
Replaced throttle body gasket and cleaned throttle body
Replaced entire plastic intake manifold
New AC Delco MAF sensor
Cleaned MAP sensor
Smoke tested via intake, didn't find any obvious leaks

I've been running a Range AFM disable device since about 104k miles when I got the truck

The fact the first run on a fresh intake manifold showed no misfires and replacing/cleaning the MAF seemed to temporarily improve things kept leading me to thinking this is a vacuum leak. I tried re-seating and re-torquing manifold with aviation gasket sealer on the metal surfaces as they looked a bit beat up in places but no change there either. Thought could be some minor vacuum leak that closes up when the engine bay is warm and expanded, but maybe the cold start misfires and cylinder 8 misfire are unrelated, although the 8 misfire is bad on cold start too.

1. Is it normal to see a bunch of random misfires until the engine warms up? Was thinking it could be to do with a plastic intake on an aluminum manifold and they're both heating up differently or plastic intake warps after some heat cycles, but silicone intake gasket should account for that
2. What's up with the minor misfire on cylinder 8 on throttle lift? Not sure where to go from here diagnosing this - could that be some worn cam/lifter/engine issue etc, although it's weird it only happens when lifting the throttle. Possible engine issue, vacuum leak somewhere else?

I'm new to GMs and truck motors, maybe this is normal and not worth worrying about but I'd like to see 0 misfires and get it running perfect

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please. If we can't see it, it does not exist.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

Sorry to read that your issue still exists. I'll bet that your parts cannon is red hot from the constant use, trying to resolve this issue. Let's see what other folks in this Forum have to suggest.

 

[Marky Dissod]

Y'know what I did NOT see? Check your grounds, especially the ones that ground out the passenger side coils rail and injectors rail.
When you replaced the injectors, did that include replacing the O-rings?
Tried running a 'Poly Ether Amine'-rich fuel system cleaner (like Chevron w/ Techron or BG44K) at doublee strength (not kidding, 2 bottles per entire tankful)?

 

[zinc]

Resistance on passenger side ground on back of engine has 0.1-0.2 ohm resistance and 0.1v voltage when running, access is tight but was able to get a 13mm back there and tighten it up a tiny bit, so seems good. Couldn't access the driver side coil/rail ground with the intake on, the one of the front of the engine similarly reads fine. Cleaned up negative battery terminal too.

Yes, new injectors had all new O-rings on etc.

I have not run PEA fuel system cleaner yet.

 

[Doubeleive]

where did you source your new injectors? and were they oem or?

 

[zinc]

Injectors were from ACDelco from Summit: https://www.summitracing.com/parts/ADO-12742701

Everything so far has either been genuine GM parts or OEM manufacturer.

 

[Doubeleive]

Injectors were from ACDelco from Summit: https://www.summitracing.com/parts/ADO-12742701

Everything so far has either been genuine GM parts or OEM manufacturer.

What does a scanner show (live data) on misfire counts, per cylinder/injector/coil

 

[zinc]

What does a scanner show (live data) on misfire counts, per cylinder/injector/coil

Copying from original post:

Reading misfire counts when running, I can see that on cold start there are a random misfires on cylinders 1-7 with counts between 1 and 4, but cylinder 8 is the worst and will see misfire counts up to 20ish.

Once running and warm, there are no misfires EXCEPT on cylinder 8, and this only occurs consistently when letting off the gas pedal or sometimes slowing down at light and the count is almost always 2, but sometimes 4 or 6. The misfire doesn't happen with the throttle open at all, only seems to occur when you snap the throttle closed after being open a decent amount e.g. on the highway when you lift.

I have a bluetooth ODB scanner with TorquePro, I'll see if I can do a screen recording of a cold start

 

[zinc]

Here's some live data recordings, misfires are on the top right, I would ignore fuel trims mostly because I have an unknown mix of 87, 91 and E85 in there which would affect those.

Cold start until the cats warm up/revs drop, looks fine until the revs drop then seem some misfires across the board - is this normal or not?

Driving immediately after cold start, note misfires on pretty much all cylinders until it warms up, cylinder 8 is the worst - is this normal or not?

Driving once warmed up, this is a single pull up to about 2500 rpm then snapping the throttle shut, notice a short time after throttle shut get a single 2 misfire on cylinder 8. Cylinder 8 is fine as long as you have your foot on the gas. All other cylinders read zero once warm.

So is it normal to see this level of misfires when the engine is cold?

 

[zinc]

I re-seated the intake then later tightened up the intake bolts from the recommended 89 in lbs to 110 in lbs and it improved the cylinder wide misfires on cold start. Also got to check the driver side engine ground on the back of the engine and it was a bit loose.

Still wasn't perfect on cold start, checked the exhaust manifold bolts when cold and some were pretty loose. Pretty sure the engine was swapped out with a re-conditioned one at some point, I wonder if they didn't torque things up or they just worked loose over time.

STILL have the same issue of 2 misfires on 8 when closing the throttle on the highway.

Also tried to do a cylinder leakdown test on 8 but given the poor access I couldn't screw in the rubber hose into the spark plug socket enough and it was leaking from the spark plug. Might need to clean up those threads or get a different adapter I can fit in there to seal up.

 

[zinc]

Well turns out it was leaking from the spark plug the whole time.

I always thought that the cylinder 8 plug got tight a little quicker than the others but I didn't want to force it and it did seem to thread in without too much trouble. In hindsight I think it wasn't even close to being properly seated.

Used a limited access spark plug thread chaser (since 8 is tough to get to) to clean up the threads, took about 3h applying grease, threading it in, backing off when it got tight and cleaning the tool with brake cleaner and compressed air in between re-greasing. The very lower threads were coked up from the misfires so took a while to work through. Also a few broken spark plug threads that must have been jammed in there from the previous owner made their way out.

Threaded a brand new plug in and is pretty much running perfect, no misfires on cold start or on 8, just the odd normal random single misfire on other cylinders.

This is the tool: https://www.autozone.com/p/duralast-thread-repair-tool-85-309/1266635

 

[BigDogYJ]

Glad you got it figured out. Sounds like you got a lot of new parts now thanks to that spark plug.

Is the odd random misfire across all cylinders actually normal?
I've been trying to figure out a semi rough idle with occasional hiccup that feels like a misfire but never get any counts via obd or CEL on our 2013 5.3. I have not got as far down the parts rabbit hole as you, but I'm on the verge of it. I might recheck the spark plugs I replaced a little over a year ago to see if something is not right there.

just the odd normal random single misfire on other cylinders.

 

[zinc]

Yeah can't be too mad having all new parts in there!

I have read multiple times that the odd misfire is normal and expected, they won't trigger any codes or CEL. I'm not sure how many exactly is considered "normal" though, I haven't found any actual numbers.

I'm surprised you don't see any counts via ODB. If it's actually mis-firing to the point you can feel it I can't believe they'd be showing zero. Low counts of 1 or 2 aren't really noticeable in the cabin for me. Are you definitely reading the GM specific misfire counter? On TorquePro it's labelled [GM]Misfire Cyl.X Current and there's another one called History but that should stay zero if the mis-fires are infrequent enough.

If it's just a vibration in the cabin did you check the engine mounts? They might be shot given the mileage

 

[Fless]

Glad you got it figured out. Sounds like you got a lot of new parts now thanks to that spark plug.

Is the odd random misfire across all cylinders actually normal?
I've been trying to figure out a semi rough idle with occasional hiccup that feels like a misfire but never get any counts via obd or CEL on our 2013 5.3. I have not got as far down the parts rabbit hole as you, but I'm on the verge of it. I might recheck the spark plugs I replaced a little over a year ago to see if something is not right there.

Sometimes a crankshaft position correlation will correct random misfires. Easily and quickly done with a capable scanner.

 

[BigDogYJ]

I haven't used TorquePro. I have a Tech2 and an autel. neither of them show anything in the misfire counters or in the misfire graphic on the tech2. I'll start with my spark plugs first to be sure before I work my way up. For me it runs great on the highway driving. but just when stopped at idle. definitely more noticeable in gear. My Brother in law has the same exact rig nearly the same miles. I checked his the other day noticed some slight vibration (maybe motor mounts) but definitely smooth compared to mine. I'll take a look at spark plugs and motor mounts. Have a trip out of town ~1k miles this weekend. I'll check those before we leave.

 

[zinc]

If it feels worse in gear, could be the gearbox? How is it in neutral?

I'd be surprised if it was misfiring but not reporting it. If I understood my research on these engines the crank position sensor is used to detect when the crank moves lined up with the timing of some cylinder firing. If the crank isn't getting a "push" when some cylinder is meant to be firing that's a clear signal of a misfire.

This seems it should be much more accurate than the knock sensor which is just a vibration sensor stuck to the side of the engine which can report false knock if something else is loose/vibrating near the sensor and the sensor thinks it's knock.

If the crank position sensor is good (and I expect you'd have much worse problems if it wasn't) then I'd be inclined to think no reported misfire = no misfires and there's probably something else going on (e.g. gearbox or motor mounts).

 

[Doubeleive]

you might try disconnecting coil packs 1 at a time and see if you can find the problem that way, you have to be able to feel it though.
disconnecting a coil will make it rougher, but at the same time smoother if that one is funky.
just something to check if you are not getting misfire counts or codes but you can feel it, that could be a weak coil pack

 

[BigDogYJ]

you might try disconnecting coil packs 1 at a time and see if you can find the problem that way, you have to be able to feel it though.
disconnecting a coil will make it rougher, but at the same time smoother if that one is funky.
just something to check if you are not getting misfire counts or codes but you can feel it, that could be a weak coil pack

Good point. I did use the tech2 to shut off individual cylinder inejctors and it definitely got worse like an expected dead miss for all 8 cylinders. So it was hard to really notice any difference. Unfortunately it’s a random occurrence so hard to reproduce. I have to admit, im hyper sensitive to this kind of thing. Most people that ride with me never notice anything until I point it out. But i know it’s not right so im gonna keep chasing it down.

 

[Doubeleive]

Good point. I did use the tech2 to shut off individual cylinder inejctors and it definitely got worse like an expected dead miss for all 8 cylinders. So it was hard to really notice any difference. Unfortunately it’s a random occurrence so hard to reproduce. I have to admit, im hyper sensitive to this kind of thing. Most people that ride with me never notice anything until I point it out. But i know it’s not right so im gonna keep chasing it down.

well that's a good thing, many people cannot detect the minor differences. Doing what I described is how I found 3 out of 8 poorly performing coil packs, no codes, no misfires but I could detect it by spidey sense I suppose.
if you are sensitive enough you should be able to compare how one cylinder sounds compared to the one next to it by disconnecting the coil packs one at a time, switching back n forth. otherwise unless you find it some other way a scope may be needed.
you have to be close and you have listen carefully. even if it is not the coil pack you should be able to narrow down the cylinder/s that are "off beat"
you ever see one of those "soothing" sound devices that are supposed to help you sleep by making wind or rain sounds? ya those things sound fake as hell and would drive me insane.
parking by a wall may help also because it reflects the sound
It probably wouldn't hurt to install a new air filter, clean the maf, clean the throttle body and use some injector cleaner for a few tank fulls. sometimes that is all these things need to smooth out the idle. I would def do that before tearing into it and throwing parts at it.